1. Size effect on bending strength and failure modes of finger-jointed timberMohammad Derikvand, Michael David Burnard, Donya Bazyar Khoshroodi, Rok Barbič, Marko Vouk, Andreja Kutnar, 2025, original scientific article Abstract: Finger-jointed timber boards for cross-laminated timber production are typically assigned the same characteristic bending strength (fm,j,k) if produced from the same strength class, regardless of differences in their cross-sectional dimensions. To validate the relevance of this approach, this study investigated the effects of cross-sectional dimensions on the bending performance of finger-jointed timber produced from spruce (strength class: C24). A large industrial dataset of 1100 specimens, with seven thicknesses (ranging 20 to 40 mm) and variable widths, were statistically analyzed. The bending tests were performed with a constant span-to-depth ratio (l/h = 18), meaning thinner specimens had a shorter test span. Bending strength was modeled with Bayesian multilevel linear model, and the proportions of three quality-control failure modes (joint interface failure, joint base failure, and outside-joint failure) were analyzed with a zero-one-inflated Dirichlet regression. Based on the results, all groups with n ≥ 100 exceeded the declared fm,j,k of 27.6 MPa, with characteristic strengths of 43.7 MPa (+58.3 %), 40.0 MPa (+45.0 %), and 38.3 MPa (+38.8 %) for the 20-, 30-, and 40-mm thickness groups, respectively. Thinner specimens demonstrated higher bending strength with convincing evidence (pairwise contrasts PD = 100 % with 95 % HDPIs entirely below zero), while width had no credible effect (PD < 95 %). Dirichlet regression revealed shifts in failure mode proportions with varying strength. Higher bending strengths were associated with a higher proportion of joint interface failure. Outside‑joint failure was observed with a higher proportion in lower-strength and thicker specimens. Overall, assigning uniform fm,j,k to various cross-sectional dimensions proved to give adequate safety margins. Beyond the uniform fm,j,k, however, statistical evidence of a size effect on both bending strength and failure modes was observed. The magnitude of the observed size effect reflects the combined influence of increasing thickness and test span under the current quality control bending test regime, which means a coupled change in stressed volume and geometry rather than a pure cross-section scaling effect. These findings are relevant to flatwise four-point bending tests on finger-jointed boards from industrial production made with visually graded C24 spruce with thickness ranging 20 to 40 mm.
Keywords: wood, finger joint, strength
Published in RUP: 18.11.2025; Views: 301; Downloads: 5
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2. Push-out resistance of threaded wooden dowels in cross-laminated timber with hardwood cross-layerLei Han, Mohammad Derikvand, Jakub Michal Sandak, Martin Hladnik, Dick Sandberg, Andreja Kutnar, 2025, original scientific article Abstract: Europe is experiencing a shift in timber resource availability, with a decline in softwoods and an increase in hardwoods, creating a need to explore the utilisation of hardwoods, particularly in construction, where softwoods have traditionally been predominant. This research aimed to assess the potential of using hardwood in hybrid, doweled cross-laminated timber (DCLT) fabricated with pre-dried threaded beech dowels. The objective was to evaluate the push-out resistance of beech dowels in a three-layer DCLT with spruce in the outer longitudinal layers and beech, poplar, or spruce in the cross-layer. The DCLT specimens were conditioned at various temperatures and relative-humidity (RH) levels before the push-out test. The specimens with beech cross-layer exhibited a higher push-out resistance than those made entirely of spruce. In contrast, using poplar in the cross-layer showed no considerable difference in push-out resistance compared to using spruce. Exposure to 70 °C and 10% RH for three days decreased the push-out resistance in all groups due to extreme shrinkage of the lamellae, whereas exposure to 20 °C and 65% RH for either 60 days or 240 days resulted in higher push-out resistance for DCLT with a beech cross-layer.
Keywords: DCLT, underutilised species, hardwood, dowel
Published in RUP: 12.09.2025; Views: 454; Downloads: 11
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4. Extraction and identification of polyphenols from spruce bark using HPLC-DAD-ESI-MS/MSEsakkiammal Sudha Esakkimuthu, Ana Miklavčič Višnjevec, Petra Jenuš, Aleksander Učakar, Črtomir Tavzes, David Brian DeVallance, Andreja Kutnar, Kelly Peeters, 2021, published scientific conference contribution abstract Keywords: polyphenols, spruce bark, paper industry
Published in RUP: 26.10.2021; Views: 2884; Downloads: 13
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6. Optimization of polyphenols extraction from spruce barkEsakkiammal Sudha Esakkimuthu, Kelly Peeters, Ana Miklavčič Višnjevec, Črtomir Tavzes, David Brian DeVallance, Andreja Kutnar, 2021, published scientific conference contribution abstract Keywords: bark, polyphenols, extraction, HPLC-DAD-ESI-MS/MS
Published in RUP: 24.06.2021; Views: 2898; Downloads: 50
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7. Electric guitar neck from densified poplar? : experimental and numerical analysisVáclav Sebera, Marica Mikuljan, Michael Nicolas Mrissa, Andreja Kutnar, Rok Prislan, Jaka Gašper Pečnik, 2021, published scientific conference contribution abstract Keywords: densified poplar, orthotropic material model, finite element analysis, electric guitar, acoustics
Published in RUP: 24.06.2021; Views: 4703; Downloads: 60
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